Low Energy Electromagnetic Physics

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Low Energy Electromagnetic Physics

• Maria Grazia Pia
• INFN Genova
• Maria.Grazia.Pia_at_cern.ch
• on behalf of the Low Energy Electromagnetic
  Working Group

http//www.ge.infn.it/geant4/lowE/
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Overview of physics

• Compton scattering
• Rayleigh scattering
• Photoelectric effect
• Pair production
• Bremsstrahlung
• Ionisation
• Polarised Compton
• atomic relaxation
• fluorescence
• Auger effect
• following processes leaving a vacancy in an atom

• In progress
• More precise angular distributions (Rayleigh,
  photoelectric, Bremsstrahlung etc.)
• Improved PIXE
• Development plan
• Driven by user requirements
• Schedule compatible with available resources

• in two flavours of models
• based on the Livermore Library
• à la Penelope

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Advanced Examples
Low Energy Electromagnetic Physics

• Stefano Agostinelli
• Henrique Araujo
• Pablo Cirrone
• Giacomo Cuttone
• Maria Catarina Espirito Santo
• Franca Foppiano
• Stefania Garelli
• Patricia Goncalves
• Alex Howard
• Ana Keating
• Susanne Larsson
• Jakub Moscicki
• Michela Piergentili
• Alberto Ribon
• Giovanni Santin
• Bernardo Tome

• Stéphane Chauvie
• Susanna Guatelli
• Vladimir Ivanchenko
• Francesco Longo
• Alfonso Mantero
• Barbara Mascialino
• Petteri Nieminen
• Luciano Pandola
• Sandra Parlati
• Luis Peralta
• Andreas Pfeiffer
• Maria Grazia Pia
• Pedro Rodrigues
• Simona Saliceti
• Andreia Trindade
• Paolo Viarengo

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Activities

• Recent physics developments
• Testing
• Open issues and concerns
• Plans

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Processes based on the Livermore Library

• Physics models are stable
• Small improvements in parameterisations
• Main effort on systematic validation
• Photon cross sections, stopping powers,
  fluorescence done
• Next round (already started) final state
  distributions
• Feedback from users
• Complementary to our own validation
• Major project for the future design iteration
• Needs stable physics models, sound physics and
  regression testing system
• Performance is an issue
• Will be addresses together with the design
  iteration, based on quantitative evaluations

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Recent development Penelope processes
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Processes à la Penelope

• Physics models by F. Salvat et al., implemented
  in a FORTRAN Monte Carlo code
• the physics models have been specifically
  developed and a great care was devoted to the low
  energy description (atomic effects, etc.)
• the (declared) lower limit is 100 eV
• The whole physics content of the Penelope Monte
  Carlo code has been re-engineered into Geant4
  (except for multiple scattering)
• processes for photons June 2003 release, for
  electrons December 2003 release (Luciano
  Pandola)
• Alternative approach w.r.t. the processes based
  on the Livermore Library

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Gamma conversion
Mean free path in Si
e- angular distr. 5 MeV g on Si
log (mfp/cm)
Livermore-based Penelope
Livermore-based Penelope
log (Energy/MeV)
Angle (degrees)
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Rayleigh scattering
Mean free path in Pb
Angular distr. 100 keV g on Pb
log (mfp/cm)
Low Energy Penelope
Low Energy Penelope
log (Energy/MeV)
Angle (degrees)
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Photoelectric effect
Mean free path in Cu
Mean free path in water
log (mfp/cm)
log (mfp/cm)
Low Energy Penelope
Low Energy Penelope
log (Energy/MeV)
log (Energy/MeV)

• The cross sections are read from the database
• Interfaced with Livermore-based atomic relaxation

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Compton scattering
e- angular distr. 1 MeV g on water
Mean free path in water
log (mfp/cm)
Low Energy Penelope
Low Energy Penelope
Angle (degrees)
log (Energy/MeV)

• Analytical parametrisation for the cross section
• The model also predicts which atomic level is
  ionised
• ? fluorescence generation (not present in
  Livermore-Compton)

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Bremsstrahlung (electrons)
g angular distr. 1 MeV e- on Pb
g energy distr. 1 MeV e- on Pb
Low Energy Penelope
Low Energy Penelope
Angle (degrees)
Relative g energy

• g energy spectrum f(Z,Eel) ? database (as in
  G4LowEnergyBremsstrahlung, but 32 points instead
  of 15)
• Also the angular distribution is data-driven
• Recent developments by V. I. not shown here

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Bremsstrahlung (positrons)
Mean free path in water

• It is assumed
• g(Z,E) ? parameterised correction function,
  independent from the g energy W

log (mfp/cm)
Electrons Positrons
log (Energy/MeV)
The g energy spectrum and the angular
distribution are the same as for electrons, only
the cross section changes
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Validation

• Relative comparison LowE-Livermore/Penelope only
  for curiosity
• helpful to understand effects of different
  modeling approaches
• and to identify software bugs!
• Validation against reference data
• LowE-Livermore and Penelope processes both
  subject to the same validation process
• See EM Validation talk by Barbara on Tuesday

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New branch of development Precise angular
distributions
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Bremsstrahlung Angular Distributions
Three LowE generators available in GEANT4 6.0
release G4ModifiedTsai, G4Generator2BS and
G4Generator2BN G4Generator2BN allows a correct
treatment at low energies (lt 500 keV)
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Bremsstrahlung Angular Distributions

• Open issues and news
• Large initialization time for G4Generator2BN (see
  Physics Manual for details)
• use of pre-calculated data (reduces
  initialization time to zero)
• introduced in Geant4 6.1
• Switching mechanism between different generators
• design iteration for final state needed
• time scale for re-implementation and test under
  discussion

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Photoelectric Angular Distributions

• Current status of photoelectric angular
  distributions in GEANT4
• G4 LowE and LowE PENELOPE processes
• The incident photon is absorbed and one electron
  is emitted in the same direction
• as the primary photon
• G4 Standard (à la GEANT3)
• The polar angle of the photoelectron is sampled
  from an approximate Sauter-Gavrila
• cross-section (for K-shell)
• PENELOPE
• The polar angle is sampled from K-shell
  cross-section derived from Sauter
• The same cross-section is used for other
  photoionisation events
• EGSnrc Controlled by a master flag IPHTER
• IPHTER 0 (similar to G4 LowE)
• IPHTER 1 (Sauter distribution valid for K-shell)

Both assume that azimuthal angle distribution is
uniform (no polarisation)
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Photoelectric Angular Distributions
in progress

• Sauter formalism is valid for light-Z, K-shell
  photoelectrons and non-polarised photons
• In progress use a more generalized approach
  based on Gavrila theory
• Valid for all elements
• For photoelectrons emitted from K and L shells
  also includes the effect of the polarisation of
  the incident photon

This enhancement is of significance importance
for the design of experiments that aim to measure
the polarisation of X-rays emitted from black
holes and neutron stars
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New development PIXE
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PIXE

• Calculation of cross sections for shell
  ionisation induced by protons or ions
• Based on a theoretical model for the calculation
  of cross sections
• M. Gryzinski, Two-Particle Collision. I. General
  Relations for Collisions in the Laboratory
  System,   Phys. Rev. vol. 138, no. 2A, 19 April
  1965
• M. Gryzinski, Two-Particle Collision. II. Coulomb
  Collisions in the Laboratory System of
  Coordinates, Phys. Rev. vol. 138, no. 2A, 19
  April 1965
• Implementation in Geant4 in 2002
• Verified to be intrinsically inadequate
• New data-driven model
• based on evaluated data library by Paul Sacher,
  1989 (compilation of experimental data
  complemented by calculations from EPCSSR model by
  Brandt Lapicki)
• Incident proton energy between 5 KeV and 500 MeV
• Elements from C to U
• Generation of X-ray spectrum based on EADL
• Uses the common de-excitation package

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PIXE Cross section model

• Fit to Paul Sacher data library results of the
  fit are used to predict the value of a cross
  section at a given proton energy
• allow extrapolations to lower/higher energy than
  data compilation
• First iteration, Geant4 6.2 (June 2004)
• The best fit is with three parametric functions
  for different groups of elements
• 6 Z 25
• 26 Z 65
• 66 Z 99

• Second iteration, Geant4 7.0
• Refined grouping of elements and parametric
  functions, to improve the model at low energies

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Quality of the PIXE model

• How good is the regression model adopted w.r.t.
  the data library?
• Goodness of model verified with analysis of
  residuals and of regression deviation
• Multiple regression index R2
• ANOVA
• Fishers test
• Results (from a set of elements covering the
  periodic table)
• 1st version (Geant4 6.2) average R2 0.998
• 2nd version (Geant4 7.0) average R2 improved to
  0.999 at low energies
• p-value from test on the F statistics lt 0.001 in
  all cases

Test statistics
Fisher distribution
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PIXE status and future developments

• First implementation for protons, K-shell
• Geant4 6.2
• preliminary model
• Second iteration
• improved model currently under test
• Geant4 7.0
• Third iteration protons, L-shell
• time scale subject to availability of resources
• Fourth iteration ions, K-shell
• compilations of cross-sections limited to K-shell

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Ongoing...

• Regular maintenance and improvements in many
  areas
• improved, precise calculation of range for
  hadrons and ions
• extension of parameterised models for hadrons up
  to 8 Mev
• improved treatment for some materials (i.e.
  graphite)
• etc.
• The physics testing project helped fixing bugs,
  identifying small problems, improving many
  details
• Thanks to systematic and quantitative tests

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Lower energy extensions

• Old requirement (still from RD44)
• Recurrent in interactions with users
• Last User Workshop requirement to reach 10 eV
• We are interested (of course)
• We are aware it is difficult
• An entire new set of phenomena to take into
  account
• Are there any data available? How good are they?
• Project for lower energy extensions currently
  explored together with ESA
• More at next Geant4 Workshop

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Current major activity Validation
See Barbaras talk on Tuesday
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Main concern SPI

• Software Process Improvement
• Huge investment in SPI
• Excellent response from most of the WG
• Many young collaborators grown to appreciate
  rigorous SP
• But also occasional spells of SPD
• Coding without design
• Maintaining private code, dropping regular
  maintenance
• Undocumented test cases
• Physics/algorithm documentation lies in the mind
  of individual developers
• Difficult change management
• etc.
• Peer reviews
• We would like to do more
• Very much needed code review
• Limitation time availability and geographical
  spread

but not worse than the rest of Geant4!
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Summary

• Validation
• Major effort, with significant results
• Highlighted some problems (solved or solutions
  planned)
• Triggered various physics improvements
• Sound testing system essential for planned design
  iteration
• Recent developments
• Penelope re-engineering
• Precise angular distributions (Bremsstrahlung,
  photoelectric)
• PIXE
• Outlook
• Lower E requirements to be addressed
• Validation of existing models
• Design and code reviews
• SPI (continuous education to a rigorous software
  process)